Executive Insight, Nuclear

Risk-based Assessments slow Aging Process for Nuclear Plants

In the United States, 99 nuclear reactors provide nearly 20 percent of the country’s electricity. However, the average age of these reactors is approximately 36 years old. Due to rigorous maintenance and safety standards, many plant operators expect to continue safely running their nuclear energy plants for another 25 to 40 years, extending plant operating periods to between 60 and 80 years in total. This longevity is possible because the industry excels at taking a solid, well-run plant and ensuring it maintains its standards or, in many cases, improves in terms of operating capacity, reliability and efficiency.

As these plants age, their repair and maintenance becomes increasingly important, especially as specific components begin to need special attention. To keep plants operating, utilities need to maintain everything from the screws and bolts to major reactor coolant system components such as reactor vessel closure heads (RVCHs) and reactor vessel internals. The best way to maintain a plant is to identify repairs early and find effective solutions that reduce repair time. As the nuclear energy industry implements smart and effective maintenance and repair strategies, it supports the continued efficient and reliable operation of nuclear energy plants across the country and around the world.

Evaluating a Range of Factors to Determine Effective Maintenance and Repair Strategies

A variety of factors drive operators’ decisions regarding which specific components need special attention and when. In some cases, regulatory guidance drives these decisions, such as with reactor vessel internals through license renewal commitments as part of the Electric Power Research Institute’s MRP-227. Meanwhile, other maintenance issues are driven more by plant reliability and the cost of operations, such as is the case with steam generator replacements.

Prioritization is the most important – determining where and when to expend the effort and resources on repairs or replacements. Each utility and plant must make these decisions based on their own operational data and cost analysis. However, since there are so many drivers in the business model, deciding which approach to take can be complicated and it can be different for each utility.

For instance, the regulated and unregulated fleets approach the business case differently. Plant operators in the unregulated fleet tend to aim for low-cost solutions such as innovative maintenance and mitigation techniques and technologies that allow them to safely maximize their asset value. By comparison, plant operators in the regulated fleet tend to take more traditional approaches such as replacing components.

A classic example of this market factor in action is the approach two different utilities took to combat a type of degradation commonly found in RVCHs: primary water stress corrosion cracking. One utility in a regulated market simply replaced the entire RVCH – an effective solution that can support the plant’s operation for another 40 years. Meanwhile, a different utility operating in an unregulated market decided to use ultra-high-pressure cavitation peening instead. This mitigation technique treats RVCH penetrations for primary water stress corrosion cracking, while leaving the original component in place. As a result, the RVCH can operate for about 20 more years at only one-tenth the cost of a replacement. The mitigation technique can be applied a second time 20 years later, extending the component’s life by another 20 years.

While both of these approaches effectively address primary water stress corrosion cracking, mitigating the problem instead of replacing the component could make the difference between continued operation of a plant or shutting down a vital asset that provides reliable, low-carbon electricity.

Making Smart Decisions with Risk-based Assessment

This is where risk-based assessment comes in. This simple process helps utilities and plant operators determine where the money and energy for repairs and replacements will be most effective. It also helps vendors, like AREVA NP, figure out which approaches can be offered to utilities to help solve issues that arise due to aging components.

Risk-based assessment depends on data gathered during regular component inspections. The high quality of these inspections is critical to ensure the best decisions are made as a result. New inspection techniques such as phased array ultrasonic testing allow utilities to uncover aging-related issues early. These techniques allow inspectors to identify even the smallest indications on a component and get encoded data outputs that map the exact location of the issue. Then, that data is fed into 3-D software that allows the engineering team to get a detailed picture of what is happening to the component.

This level of granularity allows plant operators to identify potential problems before they turn into critical issues and to find lower-cost methods of responding to or mitigating the issue. For example, a careful ultrasonic inspection of a plant’s cables could reveal some degradation of the cable casing. However, if caught early enough, plants can avoid a full-scale, expensive cable replacement and instead simply inject a new cable lining to encase the small signal and high voltage cables inside the plant.

One example of a risk-based assessment that plant operators can apply is probabilistic structural analysis. This approach allows utilities to understand the uncertainty present in a future state of the plant, in turn allowing them to more effectively plan what needs to be done during scheduled maintenance outages. Using probabilistic structural analysis, utilities can develop statistical confidence in an asset management plan for replacing certain structures, systems and components, including various quality assurance and maintenance parts.

Risk-based assessments enable utilities to quantify the risk of needing a component replacement far in advance of actually experiencing an issue. This allows utilities to integrate these actions, whether a mitigation or replacement approach, into the overall plant plan in a controlled manner, adjusting cost expectations and performance predictions far in advance. Armed with such information and insight, plant owners can transition from a reactive to a proactive, preventive maintenance stance.

One good, practical example of probabilistic structural analysis in action is how the application of this approach has been applied to finding broken or missing baffle-to-former bolts at a number of U.S. reactors. Baffle-to-former bolts are large stainless steel fasteners used to connect large steel sheets to a steel structure inside the reactor core. Because of their location in the core, baffle-to-former bolts are inaccessible by all inspection techniques but ultrasonic testing.

The probabilistic structural analysis technique uses data gathered through an ultrasonic inspection, looking at how a cross-section of the bolts is performing. This exam indicates bolt condition and provides the quantity and location of any bolt failures. Then, that data is fed through a simulation, comparing the inspection data to that of the overall industry and historic bolt degradation data from the specific plant. Combining this range of data means personnel can project future degradation. Using this technique, thousands of future patterns of degraded bolts are evaluated per minute against the design basis, allowing the plant operator to evaluate all of the uncertainty present in a future state. The operator can then determine if the projected condition is acceptable, or if preemptive replacements or repairs are needed. Utilities have also been able to use this process to justify a plant-specific, 10-year re-examination interval, where generic “one-size-fits-all” guidance would not have been applicable.

Supporting the Continued Operation of the U.S. Reactor Fleet

Risk-based assessments, paired with smart engineering solutions, will allow the U.S. reactor fleet to continue operating for decades to come. Vendors constantly research and develop new tools for such issues, like the new inspection, mitigation and maintenance techniques that help plant components operate safely and efficiently for a longer period of time.

Having the ability to confidently determine what options are available and when to implement them to manage aging-related issues boosts plant operators’ confidence in the reliability and safety of their facilities, helping to ensure that today’s nuclear fleet continues to operate for decades to come.

 

About the author: Craig Ranson is Senior Vice President of AREVA NP’s Installed Base Business Unit in the United States.